The present invention relates to delay lines for acoustic waves and more particularly to a variable delay line for surface acoustic waves.
In piezoelectric SAW (surface acoustic wave) devices, the time delay of the acoustic wave between the input and the output transducers is determined by the length of the propagation path between the transducers and by the propagation velocity of the wave between the transducers. The length of the propagation path and the propagation velocity are both modified by strain induced in the propagation medium. Thus strain produces a change in the time delay and this effect can be used to vary the performance of SAW devices. In early arrangements utilizing this phenomenon, the strain was produced by applying an external mechanical stress to the propagation medium. In a piezoelectric medium strain may also be produced by applying an electric field. The use of an electric field is attractive because it would permit extremely accurate electronic tuning of SAW devices.
Various other methods of varying the time delay of SAW devices have been developed in the past. One method was to short circuit the piezoelectric field associated with the surface wave to lower the elastic stiffness to the propagation medium and thus decrease the propagation velocity. In such devices, the shorting effect was controlled with a DC electric field by changing the distance between a metal film and the wave propagating surface or by changing the surface conductivity of a silicon wafer placed in close proximity to the wave propagating surface. These devices, however, suffer from the disadvantage that some means of mechanically supporting the silicon wafer or the metal film in close proximity to the wave propagating surface is required.
Another method of controlling the time delay involves the use of a poled ferroelectric ceramic such as PZT (lead zirconate titanate). In such devices, the time delay may be changed by electrically changing the residual polarization of the ceramic substrate on which the surface wave is propagating. See for example, Thomann, U.S. Pat. No. 3,170,465. However, in devices such as Thomannn's the application of an electric field to a polarized ceramic produces an irreversible change in the polarization of the ceramic substrate and thus irreversibly changes the properties of the propagation medium and hence irreversibly changes the time delay. Such an irreversible change in the properties of the propagation medium and in the time delay will not be useful in many desired applications. Further, ferroelectric ceramics such as PZT have an undesirable hysterisis or creep characteristic. Thus, the change in time delay will show "relaxation" effects. That is, the time delay will change with time even when the polarizing electric field is held constant. Also, ferroelectric ceramics have other disadvantages such as high acoustic attenuation, lack of good surface finish, and lack of reproducible material properties.
Another method of controlling the time delay is based on the difference in the surface wave velocity for two states of polarization of a ferroelastic-ferroelectric material. The boundary between the two states of polarization in such a material is referred to as the domain wall and control of the surface wave velocity is achieved by electrically controlling the position of the domain wall. Examples of such materials are beta-terbium molybdate and beta-gadolinium molybdate. SAW devices of this type are, however, restricted to the use of such special ferroelastic-ferroelectric materials which may not always be compatible with other requirements such as high coupling coefficient, low attenuation, and availability of crystals of the required size. Another limitation of this technique is that the domain wall of these materials moves at a very slow velocity (typically a few mm/sec) so that the device has a slow speed of response.